Left takes you into the humid tropics with its mangrove swamp, coconut palms, cascading torrent, dense rainforest of teak and balsa, rubber plantation, and Malaysian and West African farmland. The land rises steeply, paths radiate in all directions. The translucent hexagons of the roof appear through the mist far above your head. Or turning right takes you into the drier, warm temperate regions. This slightly smaller biome is planted up with warm temperate plants: flowers, vineyards, citrus and olive groves such as you might find in Mediterranean countries, or California or South Africa.

Taken as a whole, the dome complex makes up the largest conservatory in the world. But the story continues outside. Those terraced, patchwork gardens descending to the floor of the crater outside, form a roofless, cool temperate biome. That's us, in Britain, cool temperate. But it's also, for instance, the Himalayas, Chile Australia or New Zealand. And in the sheltered conditions of Cornwall, warmed by the Gulf Stream and sheltered still more by being sunk below the surrounding landscape, the open landscape of Eden is as much of a paradise for plants as the even warmer, even more protected world inside the bubbles. In all cases, the point is to demonstrate how all these plants are used by us. Or to put it another way, how very stuck we'd be without them. Thankfully Eden's progenitor, Tim Smit, loathes worthiness. He hates paying money to be made to feel guilty. And he's a rock 'n' roll kind of guy. Which makes Eden a distinctly interesting proposition.

This is the most single-minded of all Britain's Millennium projects. To say that Eden is ambitious is to be guilty of understatement. One glimpse tells you that it is crazily ambitious.To say what its purpose is, is more difficult. What does it do, after all, that a hundred other botanical gardens around the world do not? Silly question. Eden is about an attitude of mind. Its science is good, but it is not a scientific institution. It is vastly entertaining, but it is not Disney. It conveys a very serious ecological message, but it does not trade in guilt. It is a prime example of post-industrial regeneration, but it bears no relationship to the Dome in Greenwich. Smit describes it as "a living theatre mounting the planet's greatest drama." With the emphasis, you soon realise, on the drama. Eden is spectacular, Eden is upbeat, Eden is optimistic. And nearly half a million people paid to visit it last year while it was still being built. They came just to gawp from the viewing gallery on the hillside above, and to take trailer rides around the rising biomes. Even as an alternatively dusty and muddy building site, Eden was a sight to behold. You're finally allowed inside for the first time on March 17, with an official opening by Prince Charles on May 15. Goodness knows what Charles will make of it: but I'd be prepared to bet that his strapping sons go for it in a big way.